Interpretive Summary: Compared to liquid formulations, granular herbicides are generally preferred for simplicity and ease of application and broad-spectrum weed control. Granular herbicides are broadcast over container shrubs, typically using hand-carried, hand-cranked spreaders. Shrub canopies intercept some fraction of the applied herbicide, while the remainder falls to the substrate surface or the space between containers. No method has been documented to describe the amount of herbicide that falls through a canopy and deposits on the substrate surface. Sticky tape, a Petri dish, and glass bottle were evaluated to determine their suitability for aiding in assessing deposits under a potted shrub. A sunken glass bottle was shown to more effectively retain granules released from 76 cm above the target area than the other collector types. Further studies were conducted using sunken glass bottles as collectors to determine the substrate surface interception of a common granular preemergence herbicide applied to two species of shrub canopy at different stages of canopy maturity. A commercial air-boom spreader was used to apply the herbicide to the treatment area. Plant species has no significant effect on the distribution pattern of herbicide granules under the plant canopies. Across all different stages of plant maturity, mean deposits were similar for both the butterfly bush and hydrangea. Plant maturity had little effect on mean herbicide deposits for the butterfly bush canopy. However, there was much more variability across stages of maturity for the hydrangea canopy. It appears that the butterfly bushes, with more narrow leaves, are more open to movement of granules compared to the larger leafed hydrangea. Because of the variability in deposits leading to lower than effective dosage of herbicide, weed control will be more variable which could impact the quality and sale of the container plant or increase production expenses because of the need for follow-up applications or manual labor for weeding.

Technical Abstract: Preemergent herbicides are commonly used to manage weeds in nurseries that produce container-grown plants. Granular products are applied over the top of a canopy, and must pass through the canopy, to reach the substrate where weed control is needed. Little guidance is available on methods for measuring the distribution of clay herbicide granules reaching the surface of a container pot substrate. A study was conducted to evaluate three different types of collectors including sticky tape, a Petri dish and a glass bottle. The glass bottle, sunken in the substrate, provided the most efficient collection of granules released from 76 cm above the substrate surface. The jar collectors were used to evaluate differences in deposits produced by an air-boom spreader in different sizes of butterfly bush (Buddleja davidii ‘Pink Delight’) and hydrangea (H. macrophylla ‘Niko Blue’) canopies. The hydrangea canopy significantly affected deposits more than the butterfly bush canopy but there was no significant difference in mean deposits on the substrate between the two plant species. The variability in granule deposits on the substrate increased with increasing hydrangea shrub size. The same variability was not observed in the butterfly bushes.